Electro-optic displays and driving methods

ABSTRACT

This invention provides methods of and related apparatus for driving an electro-optic display having a plurality of pixels. The method includes dividing the plurality of pixels into n groups, where n is an integer larger than 1, applying a full clearing waveform to at least one group of the n groups of pixels, and applying a top off waveform to cardinal pixels of the at least one group of pixels.

REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Application Ser. No.62/466,375 filed on Mar. 3, 2017.

This application is related to U.S. Pat. Nos. 5,930,026; 6,445,489;6,504,524; 6,512,354; 6,531,997; 6,753,999; 6,825,970; 6,900,851;6,995,550; 7,012,600; 7,023,420; 7,034,783; 7,116,466; 7,119,772;7,193,625; 7,202,847; 7,259,744; 7,304,787; 7,312,794; 7,327,511;7,453,445; 7,492,339; 7,528,822; 7,545,358; 7,583,251; 7,602,374;7,612,760; 7,679,599; 7,688,297; 7,729,039; 7,733,311; 7,733,335;7,787,169; 7,952,557; 7,956,841; 7,999,787; 8,077,141; and 8,558,783;U.S. Patent Applications Publication Nos. 2003/0102858; 2005/0122284;2005/0253777; 2006/0139308; 2007/0013683; 2007/0091418; 2007/0103427;2007/0200874; 2008/0024429; 2008/0024482; 2008/0048969; 2008/0129667;2008/0136774; 2008/0150888; 2008/0291129; 2009/0174651; 2009/0179923;2009/0195568; 2009/0256799; 2009/0322721; 2010/0045592; 2010/0220121;2010/0220122; 2010/0265561; 2011/0285754; 2013/0194250, 2014/0292830 and2016/0225322; PCT Published Application No. WO 2015/017624; and U.S.patent application Ser. No. 15/014,236 filed Feb. 3, 2016.

The aforementioned patents and applications may hereinafter forconvenience collectively be referred to as the “MEDEOD” (MEthods forDriving Electro-Optic Displays) applications. The entire contents ofthese patents and co-pending applications, and of all other U.S. patentsand published and co-pending applications mentioned below, are hereinincorporated by reference.

BACKGROUND

Aspects of the present disclosure relate to electro-optic displays thatdisplay in dark mode, especially bistable electro-optic displays, and tomethods and apparatus for dark mode displaying. More specifically, thisinvention relates to driving methods in dark mode, that is, whendisplaying white text on a black background, which may allow for reducedghosting, edge artifacts and flashy updates.

SUMMARY

This invention provides methods of driving an electro-optic displayhaving a plurality of pixels to display white text on a black background(“dark mode”) while reducing edge artifacts, ghosting and flashyupdates. In some embodiments, this method for driving may includedividing a plurality of pixels into n groups, wherein n is an integerlarger than 1, applying a full clearing waveform to at least one groupof the n groups of pixels; and applying a top off waveform to cardinalpixels of the at least one group of pixels.

BRIEF DESCRIPTION OF DRAWINGS

Various aspects and embodiments of the application will be describedwith reference to the following figures. It should be appreciated thatthe figures are not necessarily drawn to scale. Items appearing inmultiple figures are indicated by the same reference number in all thefigures in which they appear.

FIG. 1A shows an electro-optic display with a plurality of displaypixels where each pixel is assigned a numerical value representing anupdating sequence order;

FIG. 1B shows the display illustrated in FIG. 1A going through multipletransitions as configured;

FIG. 2A shows an electro-optic display with a plurality of displaypixels going through multiple transitions.

FIG. 2B shows a display pixel going through an edge clearing transition.

FIG. 2C shows a display pixel going through a full clearing transition.

FIG. 2D shows a display pixel not being updated.

FIG. 3 is a graphical schematic of an inverted top-off pulse, accordingto some embodiments.

FIG. 4 is a graphical schematic of an iFull Pulse by voltage and framenumber, according to some embodiments.

FIG. 5A is another electro-optic display with a plurality of pixelsgoing through multiple transitions.

FIG. 5B is a pixel map illustrating a driving scheme for updating thepixels.

FIG. 5C is an exemplary algorithm for generating the pixel mapillustrated in FIG. 5B.

FIG. 6A is a flow chart illustrating the updating of the electro-opticdisplay illustrated in FIG. 1A and FIG. 1B.

FIG. 6B are driving schemes for updating the electro-optic displayillustrated in FIG. 1A.

FIG. 7 shows another embodiment of a plurality of pixels going throughan updating sequence.

DETAILED DESCRIPTION

The present invention relates to methods for driving electro-opticdisplays in dark mode, especially bistable electro-optic displays, andto apparatus for use in such methods. More specifically, this inventionrelates to driving methods which may allow for reduced “ghosting” andedge artifacts, and reduced flashing in such displays when displayingwhite text on a black background. This invention is especially, but notexclusively, intended for use with particle-based electrophoreticdisplays in which one or more types of electrically charged particlesare present in a fluid and are moved through the fluid under theinfluence of an electric field to change the appearance of the display.

The term “electro-optic”, as applied to a material or a display, is usedherein in its conventional meaning in the imaging art to refer to amaterial having first and second display states differing in at leastone optical property, the material being changed from its first to itssecond display state by application of an electric field to thematerial. Although the optical property is typically color perceptibleto the human eye, it may be another optical property, such as opticaltransmission, reflectance, luminescence or, in the case of displaysintended for machine reading, pseudo-color in the sense of a change inreflectance of electromagnetic wavelengths outside the visible range.

The term “gray state” is used herein in its conventional meaning in theimaging art to refer to a state intermediate two extreme optical statesof a pixel, and does not necessarily imply a black-white transitionbetween these two extreme states. For example, several of the E Inkpatents and published applications referred to above describeelectrophoretic displays in which the extreme states are white and deepblue, so that an intermediate “gray state” would actually be pale blue.Indeed, as already mentioned, the change in optical state may not be acolor change at all. The terms “black” and “white” may be usedhereinafter to refer to the two extreme optical states of a display, andshould be understood as normally including extreme optical states whichare not strictly black and white, for example the aforementioned whiteand dark blue states. The term “monochrome” may be used hereinafter todenote a drive scheme which only drives pixels to their two extremeoptical states with no intervening gray states.

Much of the discussion below will focus on methods for driving one ormore pixels of an electro-optic display through a transition from aninitial gray level (or “graytone”) to a final gray level (which may ormay not be different from the initial gray level). The terms “graystate,” “gray level” and “graytone” are used interchangeably herein andinclude the extreme optical states as well as the intermediate graystates. The number of possible gray levels in current systems istypically 2-16 due to limitations such as discreteness of driving pulsesimposed by the frame rate of the display drivers and temperaturesensitivity. For example, in a black and white display having 16 graylevels, usually, gray level 1 is black and gray level 16 is white;however, the black and white gray level designations may be reversed.Herein, graytone 1 will be used to designate black. Graytone 2 will be alighter shade of black as the graytones progress towards graytone 16(i.e., white).

The terms “bistable” and “bistability” are used herein in theirconventional meaning in the art to refer to displays comprising displayelements having first and second display states differing in at leastone optical property, and such that after any given element has beendriven, by means of an addressing pulse of finite duration, to assumeeither its first or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element. It is shownin U.S. Pat. No. 7,170,670 that some particle-based electrophoreticdisplays capable of gray scale are stable not only in their extremeblack and white states but also in their intermediate gray states, andthe same is true of some other types of electro-optic displays. Thistype of display is properly called “multi-stable” rather than bistable,although for convenience the term “bistable” may be used herein to coverboth bistable and multi-stable displays.

The term “impulse” is used herein in its conventional meaning of theintegral of voltage with respect to time. However, some bistableelectro-optic media act as charge transducers, and with such media analternative definition of impulse, namely the integral of current overtime (which is equal to the total charge applied) may be used. Theappropriate definition of impulse should be used, depending on whetherthe medium acts as a voltage-time impulse transducer or a charge impulsetransducer.

The term “remnant voltage” is used herein to refer to a persistent ordecaying electric field that may remain in an electro-optic displayafter an addressing pulse (a voltage pulse used to change the opticalstate of the electro-optic medium) is terminated. Such remnant voltagescan lead to undesirable effects on the images displayed on electro-opticdisplays, including, without limitation, so-called “ghosting” phenomena,in which, after the display has been rewritten, traces of the previousimage are still visible. The application 2003/0137521 describes how adirect current (DC) imbalanced waveform can result in a remnant voltagebeing created, this remnant voltage being ascertainable by measuring theopen-circuit electrochemical potential of a display pixel.

The term “waveform” will be used to denote the entire voltage againsttime curve used to effect the transition from one specific initial graylevel to a specific final gray level. Typically such a waveform willcomprise a plurality of waveform elements; where these elements areessentially rectangular (i.e., where a given element comprisesapplication of a constant voltage for a period of time); the elementsmay be called “pulses” or “drive pulses”. The term “drive scheme”denotes a set of waveforms sufficient to effect all possible transitionsbetween gray levels for a specific display. A display may make use ofmore than one drive scheme; for example, the aforementioned U.S. Pat.No. 7,012,600 teaches that a drive scheme may need to be modifieddepending upon parameters such as the temperature of the display or thetime for which it has been in operation during its lifetime, and thus adisplay may be provided with a plurality of different drive schemes tobe used at differing temperature etc. A set of drive schemes used inthis manner may be referred to as “a set of related drive schemes.” Itis also possible, as described in several of the aforementioned MEDEODapplications, to use more than one drive scheme simultaneously indifferent areas of the same display, and a set of drive schemes used inthis manner may be referred to as “a set of simultaneous drive schemes.”

Several types of electro-optic displays are known. One type ofelectro-optic display is a rotating bichromal member type as described,for example, in U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761;6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791(although this type of display is often referred to as a “rotatingbichromal ball” display, the term “rotating bichromal member” ispreferred as more accurate since in some of the patents mentioned abovethe rotating members are not spherical). Such a display uses a largenumber of small bodies (typically spherical or cylindrical) which havetwo or more sections with differing optical characteristics, and aninternal dipole. These bodies are suspended within liquid-filledvacuoles within a matrix, the vacuoles being filled with liquid so thatthe bodies are free to rotate. The appearance of the display is changedby applying an electric field thereto, thus rotating the bodies tovarious positions and varying which of the sections of the bodies isseen through a viewing surface. This type of electro-optic medium istypically bistable.

Another type of electro-optic display uses an electrochromic medium, forexample an electrochromic medium in the form of a nanochromic filmcomprising an electrode themed at least in part from a semi-conductingmetal oxide and a plurality of dye molecules capable of reversible colorchange attached to the electrode; see, for example O'Regan, B., et al.,Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24(March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845.Nanochromic films of this type are also described, for example, in U.S.Pat. Nos. 6,301,038; 6,870,657; and 6,950,220. This type of medium isalso typically bistable.

Another type of electro-optic display is an electro-wetting displaydeveloped by Philips and described in Hayes, R. A., et al., “Video-SpeedElectronic Paper Based on Electrowetting”, Nature, 425, 383-385 (2003).It is shown in U.S. Pat. No. 7,420,549 that such electro-wettingdisplays can be made bistable.

One type of electro-optic display, which has been the subject of intenseresearch and development for a number of years, is the particle-basedelectrophoretic display, in which a plurality of charged particles movethrough a fluid under the influence of an electric field.Electrophoretic displays can have attributes of good brightness andcontrast, wide viewing angles, state bistability, and low powerconsumption when compared with liquid crystal displays. Nevertheless,problems with the long-term image quality of these displays haveprevented their widespread usage. For example, particles that make upelectrophoretic displays tend to settle, resulting in inadequateservice-life for these displays.

As noted above, electrophoretic media require the presence of a fluid.In most prior art electrophoretic media, this fluid is a liquid, butelectrophoretic media can be produced using gaseous fluids; see, forexample, Kitamura, T., et al., “Electrical toner movement for electronicpaper-like display”, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y.,et al., “Toner display using insulative particles chargedtriboelectrically”, IDW Japan, 2001, Paper AMD4-4). See also U.S. Pat.Nos. 7,321,459 and 7,236,291. Such gas-based electrophoretic mediaappear to be susceptible to the same types of problems due to particlesettling as liquid-based electrophoretic media, when the media are usedin an orientation which permits such settling, for example in a signwhere the medium is disposed in a vertical plane. Indeed, particlesettling appears to be a more serious problem in gas-basedelectrophoretic media than in liquid-based ones, since the lowerviscosity of gaseous suspending fluids as compared with liquid onesallows more rapid settling of the electrophoretic particles.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT) and E Ink Corporationdescribe various technologies used in encapsulated electrophoretic andother electro-optic media. Such encapsulated media comprise numeroussmall capsules, each of which itself comprises an internal phasecontaining electrophoretically-mobile particles in a fluid medium, and acapsule wall surrounding the internal phase. Typically, the capsules arethemselves held within a polymeric binder to form a coherent layerpositioned between two electrodes. The technologies described in thethese patents and applications include:

(a) Electrophoretic particles, fluids and fluid additives; see forexample U.S. Pat. Nos. 7,002,728; and 7,679,814;

(b) Capsules, binders and encapsulation processes; see for example U.S.Pat. Nos. 6,922,276; and 7,411,719;

(c) Films and sub-assemblies containing electro-optic materials; see forexample U.S. Pat. Nos. 6,982,178; and 7,839,564;

(d) Backplanes, adhesive layers and other auxiliary layers and methodsused in displays; see for example U.S. Pat. Nos. 7,116,318; and7,535,624;

(e) Color formation and color adjustment; see for example U.S. Pat. No.7,075,502; and U.S. Patent Application Publication No. 007/0109219;

(f) Methods for driving displays; see the aforementioned MEDEODapplications;

(g) Applications of displays; see for example U.S. Pat. No. 7,312,784;and U.S. Patent Application Publication No. 2006/0279527; and

(h) Non-electrophoretic displays, as described in U.S. Pat. Nos.6,241,921; 6,950,220; and 7,420,549; and U.S. Patent ApplicationPublication No. 2009/0046082.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display, inwhich the electrophoretic medium comprises a plurality of discretedroplets of an electrophoretic fluid and a continuous phase of apolymeric material, and that the discrete droplets of electrophoreticfluid within such a polymer-dispersed electrophoretic display may beregarded as capsules or microcapsules even though no discrete capsulemembrane is associated with each individual droplet; see for example,the aforementioned U.S. Pat. No. 6,866,760. Accordingly, for purposes ofthe present application, such polymer-dispersed electrophoretic mediaare regarded as sub-species of encapsulated electrophoretic media.

A related type of electrophoretic display is a so-called “microcellelectrophoretic display”. In a microcell electrophoretic display, thecharged particles and the fluid are not encapsulated withinmicrocapsules but instead are retained within a plurality of cavitiesformed within a carrier medium, typically a polymeric film. See, forexample, U.S. Pat. Nos. 6,672,921 and 6,788,449, both assigned to SipixImaging, Inc.

Although electrophoretic media are often opaque (since, for example, inmany electrophoretic media, the particles substantially blocktransmission of visible light through the display) and operate in areflective mode, many electrophoretic displays can be made to operate ina so-called “shutter mode” in which one display state is substantiallyopaque and one is light-transmissive. See, for example, U.S. Pat. Nos.5,872,552; 6,130,774; 6,144,361; 6,172,798; 6,271,823; 6,225,971; and6,184,856. Dielectrophoretic displays, which are similar toelectrophoretic displays but rely upon variations in electric fieldstrength, can operate in a similar mode; see U.S. Pat. No. 4,418,346.Other types of electro-optic displays may also be capable of operatingin shutter mode. Electro-optic media operating in shutter mode may beuseful in multi-layer structures for full color displays; in suchstructures, at least one layer adjacent the viewing surface of thedisplay operates in shutter mode to expose or conceal a second layermore distant from the viewing surface.

An encapsulated electrophoretic display typically does not suffer fromthe clustering and settling failure mode of traditional electrophoreticdevices and provides further advantages, such as the ability to print orcoat the display on a wide variety of flexible and rigid substrates.(Use of the word “printing” is intended to include all forms of printingand coating, including, but without limitation: pre-metered coatingssuch as patch die coating, slot or extrusion coating, slide or cascadecoating, curtain coating; roll coating such as knife over roll coating,forward and reverse roll coating; gravure coating; dip coating; spraycoating; meniscus coating; spin coating; brush coating; air knifecoating; silk screen printing processes; electrostatic printingprocesses; thermal printing processes; ink jet printing processes;electrophoretic deposition (See U.S. Pat. No. 7,339,715); and othersimilar techniques.) Thus, the resulting display can be flexible.Further, because the display medium can be printed (using a variety ofmethods), the display itself can be made inexpensively.

Other types of electro-optic media may also be used in the displays ofthe present invention.

The bistable or multi-stable behavior of particle-based electrophoreticdisplays, and other electro-optic displays displaying similar behavior(such displays may hereinafter for convenience be referred to as“impulse driven displays”), is in marked contrast to that ofconventional liquid crystal (“LC”) displays. Twisted nematic liquidcrystals are not bi- or multi-stable but act as voltage transducers, sothat applying a given electric field to a pixel of such a displayproduces a specific gray level at the pixel, regardless of the graylevel previously present at the pixel. Furthermore, LC displays are onlydriven in one direction (from non-transmissive or “dark” to transmissiveor “light”), the reverse transition from a lighter state to a darker onebeing effected by reducing or eliminating the electric field. Finally,the gray level of a pixel of an LC display is not sensitive to thepolarity of the electric field, only to its magnitude, and indeed fortechnical reasons commercial LC displays usually reverse the polarity ofthe driving field at frequent intervals. In contrast, bistableelectro-optic displays act, to a first approximation, as impulsetransducers, so that the final state of a pixel depends not only uponthe electric field applied and the time for which this field is applied,but also upon the state of the pixel prior to the application of theelectric field.

Whether or not the electro-optic medium used is bistable, to obtain ahigh-resolution display, individual pixels of a display must beaddressable without interference from adjacent pixels. One way toachieve this objective is to provide an array of non-linear elements,such as transistors or diodes, with at least one non-linear elementassociated with each pixel, to produce an “active matrix” display. Anaddressing or pixel electrode, which addresses one pixel, is connectedto an appropriate voltage source through the associated non-linearelement. Typically, when the non-linear element is a transistor, thepixel electrode is connected to the drain of the transistor, and thisarrangement will be assumed in the following description, although it isessentially arbitrary and the pixel electrode could be connected to thesource of the transistor. Conventionally, in high resolution arrays, thepixels are arranged in a two-dimensional array of rows and columns, suchthat any specific pixel is uniquely defined by the intersection of onespecified row and one specified column. The sources of all thetransistors in each column are connected to a single column electrode,while the gates of all the transistors in each row are connected to asingle row electrode; again the assignment of sources to rows and gatesto columns is conventional but essentially arbitrary, and could bereversed if desired. The row electrodes are connected to a row driver,which essentially ensures that at any given moment only one row isselected, i.e., that there is applied to the selected row electrode avoltage such as to ensure that all the transistors in the selected roware conductive, while there is applied to all other rows a voltage suchas to ensure that all the transistors in these non-selected rows remainnon-conductive. The column electrodes are connected to column drivers,which place upon the various column electrodes voltages selected todrive the pixels in the selected row to their desired optical states.(The aforementioned voltages are relative to a common front electrodewhich is conventionally provided on the opposed side of theelectro-optic medium from the non-linear array and extends across thewhole display.) After a pre-selected interval known as the “line addresstime” the selected row is deselected, the next row is selected, and thevoltages on the column drivers are changed so that the next line of thedisplay is written. This process is repeated so that the entire displayis written in a row-by-row manner.

It might at first appear that the ideal method for addressing such animpulse-driven electro-optic display would be so-called “generalgrayscale image flow” in which a controller arranges each writing of animage so that each pixel transitions directly from its initial graylevel to its final gray level. However, inevitably there is some errorin writing images on an impulse-driven display. Some such errorsencountered in practice include:

(a) Prior State Dependence; With at least some electro-optic media, theimpulse required to switch a pixel to a new optical state depends notonly on the current and desired optical state, but also on the previousoptical states of the pixel.

(b) Dwell Time Dependence; With at least some electro-optic media, theimpulse required to switch a pixel to a new optical state depends on thetime that the pixel has spent in its various optical states. The precisenature of this dependence is not well understood, but in general, moreimpulse is required the longer the pixel has been in its current opticalstate.

(c) Temperature Dependence; The impulse required to switch a pixel to anew optical state depends heavily on temperature.

(d) Humidity Dependence; The impulse required to switch a pixel to a newoptical state depends, with at least some types of electro-optic media,on the ambient humidity.

(e) Mechanical Uniformity; The impulse required to switch a pixel to anew optical state may be affected by mechanical variations in thedisplay, for example variations in the thickness of an electro-opticmedium or an associated lamination adhesive. Other types of mechanicalnon-uniformity may arise from inevitable variations between differentmanufacturing batches of medium, manufacturing tolerances and materialsvariations.

(f) Voltage Errors; The actual impulse applied to a pixel willinevitably differ slightly from that theoretically applied because ofunavoidable slight errors in the voltages delivered by drivers.

General grayscale image flow suffers from an “accumulation of errors”phenomenon. For example, imagine that temperature dependence results ina 0.2 L* (where L* has the usual CIE definition:

L*=116(R/R0)1/3−16,

where R is the reflectance and R0 is a standard reflectance value) errorin the positive direction on each transition. After fifty transitions,this error will accumulate to 10 L*. Perhaps more realistically, supposethat the average error on each transition, expressed in terms of thedifference between the theoretical and the actual reflectance of thedisplay is ±0.2 L*. After 100 successive transitions, the pixels willdisplay an average deviation from their expected state of 2 L*; suchdeviations are apparent to the average observer on certain types ofimages.

This accumulation of errors phenomenon applies not only to errors due totemperature, but also to errors of all the types listed above. Asdescribed in the aforementioned U.S. Pat. No. 7,012,600, compensatingfor such errors is possible, but only to a limited degree of precision.For example, temperature errors can be compensated by using atemperature sensor and a lookup table, but the temperature sensor has alimited resolution and may read a temperature slightly different fromthat of the electro-optic medium. Similarly, prior state dependence canbe compensated by storing the prior states and using a multi-dimensionaltransition matrix, but controller memory limits the number of statesthat can be recorded and the size of the transition matrix that can bestored, placing a limit on the precision of this type of compensation.

Thus, general grayscale image flow requires very precise control ofapplied impulse to give good results, and empirically it has been foundthat, in the present state of the technology of electro-optic displays,general grayscale image flow is infeasible in a commercial display.

The aforementioned US 2013/0194250 describes techniques for reducingflashing and edge ghosting. One such technique, denoted a “selectivegeneral update” or “SGU” method, involves driving an electro-opticdisplay having a plurality of pixels using a first drive scheme, inwhich all pixels are driven at each transition, and a second drivescheme, in which pixels undergoing some transitions are not driven. Thefirst drive scheme is applied to a non-zero minor proportion of thepixels during a first update of the display, while the second drivescheme is applied to the remaining pixels during the first update.During a second update following the first update, the first drivescheme is applied to a different non-zero minor proportion of thepixels, while the second drive scheme is applied to the remaining pixelsduring the second update. Typically, the SGU method is applied torefreshing the white background surrounding text or an image, so thatonly a minor proportion of the pixels in the white background undergoupdating during any one display update, but all pixels of the backgroundare gradually updated so that drifting of the white background to a graycolor is avoided without any need for a flashy update. It will readilybe apparent to those skilled in the technology of electro-optic displaysthat application of the SGU method requires a special waveform(hereinafter referred to as an “F” waveform or “F-Transition”) for theindividual pixels which are to undergo updating on each transition.

The aforementioned US 2013/0194250 also describes a “balanced pulse pairwhite/white transition drive scheme” or “BPPWWTDS”, which involves theapplication of one or more balanced pulse pairs (a balanced pulse pairor “BPP” being a pair of drive pulses of opposing polarities such thatthe net impulse of the balanced pulse pair is substantially zero) duringwhite-to-white transitions in pixels which can be identified as likelyto give rise to edge artifacts, and are in a spatio-temporalconfiguration such that the balanced pulse pair(s) will be efficaciousin erasing or reducing the edge artifact. Desirably, the pixels to whichthe BPP is applied are selected such that the BPP is masked by otherupdate activity. Note that application of one or more BPP's does notaffect the desirable DC balance of a drive scheme since each BPPinherently has zero net impulse and thus does not alter the DC balanceof a drive scheme. A second such technique, denoted “white/white top-offpulse drive scheme” or “WWTOPDS”, involves applying a “top-off” pulseduring white-to-white transitions in pixels which can be identified aslikely to give rise to edge artifacts, and are in a spatio-temporalconfiguration such that the top-off pulse will be efficacious in erasingor reducing the edge artifact. Application of the BPPWWTDS or WWTOPDSagain requires a. special waveform (hereinafter referred to as a “T”waveform or “T-Transition”) for the individual pixels which are toundergo updating on each transition. The T and F waveforms are normallyonly applied to pixels undergoing white-to-white transitions. In aglobal limited drive scheme, the white-to-white waveform is empty (i.e.,consists of a series of zero voltage pulses) whereas all other waveformsare not empty. Accordingly, when applicable the non-empty T and Fwaveforms replace the empty white-to-white waveforms in a global limiteddrive scheme.

Under some circumstances, it may be desirable for a single display tomake use of multiple drive schemes. For example, a display capable ofmore than two gray levels may make use of a gray scale drive scheme(“GSDS”) which can effect transitions between all possible gray levels,and a monochrome drive scheme (“MDS”) which effects transitions onlybetween two gray levels, the MDS providing quicker rewriting of thedisplay than the GSDS. The MDS is used when all the pixels which arebeing changed during a rewriting of the display are effectingtransitions only between the two gray levels used by the MDS. Forexample, the aforementioned U.S. Pat. No. 7,119,772 describes a displayin the form of an electronic book or similar device capable ofdisplaying gray scale images and also capable of displaying a monochromedialogue box which permits a user to enter text relating to thedisplayed images. When the user is entering text, a rapid MDS is usedfor quick updating of the dialogue box, thus providing the user withrapid confirmation of the text being entered. On the other hand, whenthe entire gray scale image shown on the display is being changed, aslower GSDS is used.

Alternatively, a display may make use of a GSDS simultaneously with a“direct update” drive scheme (“DUDS”). The DUDS may have two or morethan two gray levels, typically fewer than the GSDS, but the mostimportant characteristic of a DUDS is that transitions are handled by asimple unidirectional drive from the initial gray level to the finalgray level, as opposed to the “indirect” transitions often used in aGSDS, where in at least some transitions the pixel is driven from aninitial gray level to one extreme optical state, then in the reversedirection to a final gray level; in some cases, the transition may beeffected by driving from the initial gray level to one extreme opticalstate, thence to the opposed extreme optical state, and only then to thefinal extreme optical state—see, for example, the drive schemeillustrated in FIGS. 11A and 11B of the aforementioned U.S. Pat. No.7,012,600. Thus, present electrophoretic displays may have an updatetime in grayscale mode of about two to three times the length of asaturation pulse (where “the length of a saturation pulse” is defined asthe time period, at a specific voltage, that suffices to drive a pixelof a display from one extreme optical state to the other), orapproximately 700-900 milliseconds, whereas a DUDS has a maximum updatetime equal to the length of the saturation pulse, or about 200-300milliseconds.

Variation in drive schemes is, however, not confined to differences inthe number of gray levels used. For example, drive schemes may bedivided into global drive schemes, where a drive voltage is applied toevery pixel in the region to which the global update drive scheme (moreaccurately referred to as a “global complete” or “GC” drive scheme) isbeing applied (which may be the whole display or some defined portionthereof) and partial update drive schemes, where a drive voltage isapplied only to pixels that are undergoing a non-zero transition (i.e.,a transition in which the initial and final gray levels differ from eachother), but no drive voltage or zero voltage is applied during zerotransitions or null transitions (in which the initial and final graylevels are the same). As used herein, the terms “zero transition” and“null transition” are used interchangeably. An intermediate form ofdrive scheme (designated a “global limited” or “GL” drive scheme) issimilar to a GC drive scheme except that no drive voltage is applied toa pixel which is undergoing a zero, white-to-white transition. In, forexample, a display used as an electronic book reader, displaying blacktext on a white background, there are numerous white pixels, especiallyin the margins and between lines of text which remain unchanged from onepage of text to the next; hence, not rewriting these white pixelssubstantially reduces the apparent “flashiness” of the displayrewriting.

However, certain problems remain in this type of GL drive scheme.Firstly, as discussed in detail in some of the aforementioned MEDEODapplications, bistable electro-optic media are typically not completelybistable, and pixels placed in one extreme optical state graduallydrift, over a period of minutes to hours, towards an intermediate graylevel. In particular, pixels driven white slowly drift towards a lightgray color. Hence, if in a GL drive scheme a white pixel is allowed toremain undriven through a number of page turns, during which other whitepixels (for example, those forming parts of the text characters) aredriven, the freshly updated white pixels will be slightly lighter thanthe undriven white pixels, and eventually the difference will becomeapparent even to an untrained user.

Secondly, when an undriven pixel lies adjacent a pixel which is beingupdated, a phenomenon known as “blooming” occurs, in which the drivingof the driven pixel causes a change in optical state over an areaslightly larger than that of the driven pixel, and this area intrudesinto the area of adjacent pixels. Such blooming manifests itself as edgeeffects along the edges where the undriven pixels lie adjacent drivenpixels. Similar edge effects occur when using regional updates (whereonly a particular region of the display is updated, for example to showan image), except that with regional updates the edge effects occur atthe boundary of the region being updated. Over time, such edge effectsbecome visually distracting and must be cleared. Hitherto, such edgeeffects (and the effects of color drift in undriven white pixels) havetypically been removed by using a single GC update at intervals.Unfortunately, use of such an occasional GC update reintroduces theproblem of a “flashy” update, and indeed the flashiness of the updatemay be heightened by the fact that the flashy update only occurs at longintervals.

Some of the aspects of the present invention relates to reducing oreliminating the problems discussed above while still avoiding so far aspossible flashy updates. However, there is an additional complication inattempting to solve the aforementioned problems, namely the need foroverall DC balance. As discussed in many of the aforementioned MEDEODapplications, the electro-optic properties and the working lifetime ofdisplays may be adversely affected if the drive schemes used are notsubstantially DC balanced (i.e., if the algebraic sum of the impulsesapplied to a pixel during any series of transitions beginning and endingat the same gray level is not close to zero). See especially theaforementioned U.S. Pat. No. 7,453,445, which discusses the problems ofDC balancing in so-called “heterogeneous loops” involving transitionscarried out using more than one drive scheme. A DC balanced drive schemeensures that the total net impulse bias at any given time is bounded(for a finite number of gray states). In a DC balanced drive scheme,each optical state of the display is assigned an impulse potential (IP)and the individual transitions between optical states are defined suchthat the net impulse of the transition is equal to the difference inimpulse potential between the initial and final states of thetransition. In a DC balanced drive scheme, any round trip net impulse isrequired to be substantially zero.

In one aspect, this invention provides methods of driving anelectro-optic display having a plurality of pixels to display white texton a black background (“dark mode” also referred to herein as “blackmode”) while reducing edge artifacts, ghosting and flashy updates. Inaddition, the white text may include pixels having intermediate graylevels, if the text is anti-aliased. Displaying black text on a light orwhite background is referred to herein as “light mode” or “white mode”.Typically, when displaying white text on a black background, white edgesor edge artifacts may accumulate after multiple updates (as with darkedges in the light mode). This edge accumulation is particularly visiblewhen the background pixels (i.e., pixels in the margins and in theleading between lines of text) do not flash during updates (i.e., thebackground pixels, which remain in the black extreme optical statethrough repeated updates, undergo repeated black-to-black zerotransitions, during which no drive voltages are applied to the pixels,and they do not flash). A dark mode where no drive voltages are appliedduring black-to-black transitions may be referred to as a “dark GLmode”; this is essentially the inverse of a light GL mode where no drivevoltages are applied to the background pixels undergoing white-to-whitezero transitions. The dark GL mode may be implemented by simply defininga zero transition for black-to-black pixels, but also, may beimplemented by some other means such as a partial update by thecontroller.

In some embodiments, to maintain a consistent black background in theabove mentioned “dark-mode”, and to update the pixels in a display tomaintain a consistent gray tone appearance, and yet to avoid the displaybeing too flashy during the update, one may choose to program thedisplay in such a fashion that the pixels are grouped into multiplegroups and the pixels are updated one group at a time. In another word,a sub-population of the pixels are updated with a waveform at any giventime, and each pixel is visited or updated over a certain number ofupdates, thereby clearing the display of edges and other artifacts(e.g., graytone drifts) over time. This configuration allows for acomplete update or reset of the display pixels while maintain arelatively pleasant appearance (e.g., avoid being overly flashy) to auser.

FIG. 1A illustrates an exemplary setup where several sub-populations ofbackground pixels are updated or reset on a rotating per-update basis.The decision as to which sub-population of pixel may be updated or resetat any given time may be pre-determined systematically using atessellating pattern, or, statistically, with an appropriate propositionof pixels being selected randomly at each update. Shown in FIG. 1A andFIG. 1B are a dithering mask and the updated sub-population ofbackground pixels in each frames. This configuration can effectivelyreduce image graytone drifting, since all background pixels are updatefor some, every fixed number of panel updates, while only producing amild flash, or dip, in background dark state during updates. Using adithering mask as shown in FIG. 1A as an example, where every pixel isassigned a numerical value n (e.g., 1-8), and all background pixels willbe updated once every n (e.g., n=8) frames. In another word, theplurality of display pixels within a display can be divided in to ngroups, where n is a numerical value larger than 1, and the n groups ofpixels may be updated one group at a time, until all the pixels havebeen updated or reset. The sequence of which group of pixels to beupdated may be pre-determined by, for example, a computer algorithm. Inthe example presented in FIG. 1A, the pixel groups may be updatedaccording to the numerical order 1-8, but it should be appreciated thatany other update order or sequence may be applied according toapplication needs. In sonic embodiment, all groups of pixels may beupdated, in some other embodiments, certain groups of the pixels may beupdated. It should he appreciated that the size of a dithering maskcould affect image graytone drifts, update flashness, localized fatigueand/or remnant voltage. For example, making the mask size large willhave less updated pixels per frame which can result in a larger imagegraytone drift while less flashy update, localized fatigue and remnantvoltage.

Furthermore, the nature of ink dictates that a DC-imbalanced waveformmay be required to reset or update the background pixels, the details ofthe DC-imbalanced waveforms to be discussed in more detail below in FIG.3 and FIG. 4. A such DC-imbalanced waveform may be a full clearingwaveform (e.g., an iFull Pulse) or a top-off pulse (e.g., an iTopPulse). In some embodiments, a full clearing waveform may produce abetter cleaning or resetting result. However, when a full clearingwaveform is applied, such waveform may produce its own edge artifactsaround an updated pixel which can persist until those pixels arethemselves updated. As such, it may be necessary to perform edgeclearing on these updated pixels.

FIG. 2A illustrates a plurality of display pixels going through anupdating/resetting then edge clearing transition or sequence asdescribed above. As shown, some of the pixels (e.g., pixels 200, 202,204) may go through full clearing transition (e.g., an iFull pulse isapplied of the state I, as illustrated in FIG. 2C); and such pixels maygenerate some edge artifacts, and as a result, its cardinal pixels(e.g., pixels 206, 208, 210 and 212 are cardinal pixels to pixel 200)will be applied an “edge clearing” transition (i.e., an iTop pulse orstate S, as illustrated in FIG. 1B). This process can be applied to allthe pixels within the selected group and their cardinal pixels to ensureall the pixels are free of optical artifacts, as well as to produce auniform graytone throughout the display. Otherwise, the other pixels(e.g., pixels 214, 216) may stay idle (i.e., going through a nulltransition), as illustrated in FIG. 2D, they remain in an “empty” state.

FIG. 3 illustrates a graphical schematic of an inverted top-off pulse,where such waveform may be applied to “edge clear” a display pixel, asillustrated above in FIG. 2B. The iTop Pulse may be defined by twotunable parameters—the size (impulse) of the pulse (“iTop size”—i.e.,the integral of the applied voltage with respect to time) and the“padding” i.e., the period between the end of the iTop Pulse and end ofthe waveform (“iTop pad”). These parameters are tunable and may bedetermined by the type of display and its use, the preferred ranges innumber of frames are: size between 1 and 35, and pad between 0 and 50.As stated above these ranges may be larger if display performance sorequires.

In some embodiments, the iTop Pulse used in dark mode displaying may beapplied inversely (opposite polarity) to reduce ghosting, edge artifactsand flashiness when displaying in light mode as a “top-off pulse”. Asdescribed in aforementioned U.S. Patent Publication No. 2013/0194250,which is incorporated herein in its entirety, a “top-off pulse” appliedto a white or near-white pixel drives the pixel to the extreme opticalwhite state (and is the opposite polarity of the iTop Pulse, whichdrives the pixel to the extreme optical black state). Typically, thetop-off pulse is not used due to its DC imbalanced waveform. However,when used in conjunction with the remnant voltage discharging, theeffects of the DC imbalanced waveform may be reduced or eliminated andthe display performance may be enhanced. Thus, the top-off pulse is lesslimited in terms of size and application. In some embodiments, thetop-off size may be up to 10 frames and may be even greater. Further, asdescribed, the top-off pulse may be applied in place of the balancedpulse pair (“BPP”), which is a pair of drive pulses of opposingpolarities such that the net impulse of the balanced pulse pair issubstantially zero.

FIG. 4 is a graphical schematic of an iFull Pulse where voltage is onthe y-axis and frame number is on the x-axis. Each frame number denotesthe time interval of 1 over the frame rate of the active matrix module.The iFull Pulse may be defined by four tunable parameters: 1) the size(impulse) of the iFull Pulse that drives to white (“pl1” parameter); 2)the “gap” parameter, i.e., the period between the end of the “pl1” andthe “pl2” parameter; 3) the size of the iFull Pulse that drives to black(“pl2”) and the “padding” parameter—i.e., the period between the end ofthe pl2 and end of the waveform (“pad”). The pl1 represents the initialdrive to white state. The pl2 represents the drive to black state. TheiFull Pulse improves lightness error by erasing the edge artifacts thatmay be created by adjacent pixels not driving from black to black.However, the iFull Pulse may introduce significant DC imbalance. TheiFull Pulse parameters are tunable to optimize the performance of thedisplay by reducing edge artifact accumulation with minimum DCimbalance. Although all parameters are tunable and may be determined bythe type of display and its use, the preferred ranges in number offrames are: impulse size between 1 and 25, gap between 0 and 25, sizebetween 1 and 35, and pad between 0 and 50. As stated above these rangesmay be larger if display performance so requires.

FIG. 5A illustrates a plurality of pixels going through a series ofupdate cycles (e.g., 8 cycles) to update the entire set of pixels, whereeach update cycle updates only a portion of the pixels, as describedabove. FIG. 5B illustrates an exemplary pixel map matrix where eachdisplay pixel is programmed to be updated in a particular update cycle.FIG. 5C illustrates an exemplary algorithm where the pixel map of FIG.5B may be generated.

FIG. 6A illustrates an exemplary flow process where a plurality ofpixels may be firstly mapped out and subsequently updated in aparticular update cycle. Where in step 610 a desired dithering mask maybe chosen, the size of the dithering mask may depend on design goalsregarding overall display flashness, pixel fatigue and update time. Instep 612, each display pixel is assigned a numerical value, such thatdisplay pixels will be grouped according their assigned number and willbe updated one group (e.g., 8 groups as described above) at a time.Finally in step 614, when the pixels are going through the update phase,appropriate waveforms will be applied to the pixels. For example, asdiscussed above, the group of pixels that are chosen to go through theupdate will be applied an iFull pulse, while its cardinal neighbors willbe applied an iTop pulse to get rid of the edge artifacts. FIG. 6Billustrates one embodiment of an algorithm where the process illustratedin FIG. 6A may be implemented.

Alternatively, in some other embodiments, pixels chosen for a fullupdate or reset (e.g., pixels 702 and 704) may be instead applied atop-off pulse (e.g., iTop pulse) instead of a full clearing pulse (e.g.,iFull pulse), and its cardinal pixels may stay idle or be applied a nullwaveform. This setup allows for an even less flashy update of the darkbackground pixels in a dark mode operation. Due to the nature of the inkparticles, applying only a top-off pulse such as the iTop pulse insteadof a full clearing pulse can produce an even less flashy update of thedisplay pixels, while still maintain a relatively consistent graytonethroughout the display (e.g., between the updated pixels 702, 704 andthe its idle cardinal pixels).

It will be apparent to those skilled in the art that numerous changesand modifications can be made in the specific embodiments of theinvention described above without departing from the scope of theinvention. Accordingly, the whole of the foregoing description is to beinterpreted in an illustrative and not in a limitative sense.

1. A method for driving an electro-optic display having a plurality ofpixels, the method comprising: dividing the plurality of pixels into ngroups, wherein n is an integer larger than 1; applying a full clearingwaveform to at least one group of the n groups of pixels; and applying atop off waveform to cardinal pixels of the at least one group of pixels.2. The method of claim 1 wherein the applying a full clearing waveformstep further comprising applying the full clearing waveform to allgroups of pixels in a pre-determined sequence.
 3. The method of claim 1,wherein the electro-optic display is an electrophoretic display having alayer of display medium.
 4. The method of claim 4 wherein the layer ofdisplay medium is an electrophoretic medium.
 5. The method of claim 4wherein the layer of display medium is an encapsulated electrophoreticdisplay medium.
 6. The method of claim 4 wherein the electrophoreticdisplay medium comprises an electrophoretic medium comprising a liquidand at least one particle disposed within said liquid and capable ofmoving therethrough on application of an electric field to the medium.7. A method for driving an electro-optic display having a plurality ofpixels, the method comprising: dividing the plurality of pixels into ngroups, wherein n is an integer larger than 1; and applying a top offwaveform to at least one group of the n groups of pixels.
 8. The methodof claim 7 wherein the applying a full clearing waveform step furthercomprising applying the full clearing waveform to all groups of pixelsin a pre-determined sequence.
 9. The method of claim 7, wherein theelectro-optic display is an electrophoretic display having a layer ofdisplay medium.
 10. The method of claim 9 wherein the layer of displaymedium is an electrophoretic medium.
 11. The method of claim 9 whereinthe layer of display medium is an encapsulated electrophoretic displaymedium.
 12. The method of claim 9 wherein the electrophoretic displaymedium comprises an electrophoretic medium comprising a liquid and atleast one particle disposed within said liquid and capable of movingtherethrough on application of an electric field to the medium.